Try using the dock's output fed into your preamp, that should allow for more volume thru your home system. As for low output into headphones, you have to make sure your headphones are a good match for the output impedance of the iPod.
I have a Rio Karma MP3 player and it drives my home system quite satisfactorily when using the dock's output. In addition, I find that my Sennheiser 580s don't achieve very high volume (but still okay) when using the headphone output, but my Shure E2Cs can really blast at the same volume setting.
Get rid of the cheesy earbuds that Apple provides and try some Shures or Etymotics instead. Go to HeadFi.org for much more detailed info...
-RW-